The present invention relates to a method and apparatus for separating gas fractions from a gas mixture having multiple gas fractions. In particular, the present invention relates to a multistage gas separation system having uniform gas flow between each stage.
Pressure swing adsorption (PSA) and vacuum pressure swing adsorption (vacuum-PSA) separate gas fractions from a gas mixture by coordinating pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure of the gas mixture in the adsorbent bed is elevated while the gas mixture is flowing through the adsorbent bed from a first end to a second end thereof, and is reduced while the gas mixture is flowing through the adsorbent from the second end back to the first end. As the PSA cycle is repeated, the less readily adsorbed component is concentrated adjacent the second end of the adsorbent bed, while the more readily adsorbed component is concentrated adjacent the first end of the adsorbent bed. As a result, a xe2x80x9clightxe2x80x9d product (a gas fraction depleted in the more readily adsorbed component and enriched in the less readily adsorbed component) is delivered from the second end of the bed, and a xe2x80x9cheavyxe2x80x9d product (a gas fraction enriched in the more strongly adsorbed component) is exhausted from the first end of the bed.
The conventional system for implementing pressure swing adsorption uses two or more stationary adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks. However, this system is often difficult and expensive to implement due to the complexity of the valving required. Further, it is difficult to obtain a process result (e.g. yield, purity) which is not compromised by the limitations imposed by presently-available adsorbent materials. Furthermore, the conventional PSA system makes inefficient use of applied energy, because feed gas pressurization is provided by a compressor whose delivery pressure is the highest pressure of the cycle. Consequently, energy expended in compressing the feed gas used for pressurization is then dissipated in throttling over valves over the instantaneous pressure difference between the adsorber and the high pressure supply.
Numerous attempts have been made at overcoming the deficiencies associated with the conventional PSA system. For example, Siggelin (U.S. Pat. No. 3,176,446), Mattia (U.S. Pat. No. 4,452,612), Davidson and Lywood (U.S. Pat. No. 4,758,253), Boudet et al (U.S. Pat. No. 5,133,784) and Petit et al (U.S. Pat. No. 5,441,559) disclose PSA devices using rotary distributor valves whose rotors are fitted with multiple angularly separated adsorbent beds. Ports communicating with the rotor-mounted adsorbent beds sweep past fixed ports for feed admission, product delivery and pressure equalization. However, these prior art rotary devices are impracticable for large PSA units, owing to the weight of the rotating assembly. Furthermore, since the valve faces are remote from the ends of the adsorbent beds, these rotary distributor valves have poor flow distribution, particularly at high cycle frequencies. Also, the gas separation yields and purities are limited by the constraints of the adsorbent material used.
Hay (U.S. Pat. No. 5,246,676) and Engler (U.S. Pat. No. 5,393,326) provide examples of vacuum pressure swing adsorption systems which reduce throttling losses in an attempt to improve the efficiency of the gas separation process system. The systems taught by Hay and Engler use a plurality of vacuum pumps to pump down the pressure of each adsorbent bed sequentially in turn, with the pumps operating at successively lower pressures, so that each vacuum pump reduces the pressure in each bed a predetermined amount. However, with these systems, the vacuum pumps are subjected to large pressure variations, thereby reducing the efficiency of the gas separation process.
Accordingly, there remains a need for a PSA system which is suitable for high volume and high frequency production, which reduces the energy losses associated with the prior art devices, and can be more readily configured to obtain the desired process results.
According to the invention, there is provided a gas separation system and method which addresses deficiencies of the prior art.
The gas separation system, according to the present invention, includes a first adsorbent module, and a second adsorbent module coupled to the first adsorbent module. The first adsorbent module includes a first gas inlet for receiving a gas mixture, at least one bed of first adsorbent material in communication with the first gas inlet for adsorbing a gas mixture component from the gas mixture, and a first gas outlet in communication with the first adsorbent beds for receiving a first product gas therefrom. The second adsorbent module includes a second gas inlet coupled to the first gas outlet for receiving the first product gas, at least one second bed of adsorbent material in communication with the second gas inlet for adsorbing a first product gas component from the first product gas, and a second gas outlet in communication with the second adsorbent beds for receiving a second product gas therefrom. The first product gas substantially excludes the adsorbed gas mixture component, and the second product gas substantially excludes the adsorbed first product gas component. Also, the adsorbent modules are configured for transferring the first product gas between the adsorbent modules over a plurality of discrete pressure levels to maintain substantial uniformity of gas flow therebetween.
The gas separation method, according to the present invention, includes the steps of (1) providing a first adsorbent module including at least one bed of a first adsorbent material; (2) providing a second adsorbent module in communication with the first adsorbent module, the second adsorbent module including at least one bed of a second adsorbent material; (3) adsorbing a gas mixture component from the gas mixture with the first adsorbent material; (4) transferring a first product gas from between the first adsorbent module and the second adsorbent module with substantially uniform gas flow, the first product gas substantially excluding the adsorbed gas mixture component; (5) adsorbing a first product gas component from the first product gas with the second adsorbent material, and (6) extracting a second product gas from the second adsorbent module, the second product gas substantially excluding the adsorbed first product gas component.
In accordance with a preferred embodiment of the present invention, each adsorbent module comprises a rotary pressure swing adsorbent module. Each rotary pressure swing adsorbent module includes a stator and a rotor. The stator includes a first stator valve surface, a second stator valve surface, a plurality of first function compartments opening into the first stator valve surface, and a plurality of second function compartments opening into the second stator valve surface. The rotor is rotatably coupled to the stator and includes a first rotor valve surface in communication with the first stator valve surface, a second rotor valve surface in communication with the second stator valve surface.
A plurality of flow paths having adsorbent material therein are disposed in the rotors. Each of the flow paths includes a pair of opposite flow path ends. A plurality of apertures are provided in the rotor valve surfaces in communication with the flow path ends and the function compartments for cyclically exposing the flow paths to a plurality of discrete pressure levels to maintain uniformity of gas flow through the function compartments. In this manner, product gas is transferred between the adsorbent modules at the plurality of discrete pressure levels with substantially uniform gas flow, thereby reducing energy losses. Further, the first rotor can be operated at a different speed than the second rotor, and the first and second adsorbent material can be selected independently of each other so as to obtain the desired process results more readily.